A peculiar type of Cr-Ni-mineralization; cordierite-chromite-niccolite ores of Malaga, Spain, and their possible origin by liquid unmixing

Mineralogical evidence indicates that cordierite rocks and associated chromite-niccolite ores occurring in alpine-type ultramafics at Los Jarales and La Gallega in Malaga province, southern Spain, crystallized within range of liquidus temperatures below 1,300 degrees C but above 800 degrees C and at pressures below 5-10 kb. Polyphase arsenide inclusions in chromite, adjustment of chromite-arsenide boundaries during recrystallization of chromite above liquidus temperatures of arsenides, and constant ratios of chromite to arsenides suggest crystallization of chromite-niccolite rocks from immiscible chromite- rich and arsenide-rich liquids. Occluded cordierite and chromite-niccolite chain textures along contacts of cordierite and chromite-niccolite rocks, occurrence of these rocks with mutually crosscutting relations in the same veins and schlieren, and unusually high partition of V and Zn in chromite suggest liquid immiscibility of a silicate and an oxyarsenide magma. The parental magma presumably originated by partial fusion of mantle peridotites during emplacement in the crust. This magma intruded higher low-pressure levels in the peridotite and began crystallization of pyroxenites; from the residual differentiated magma a hydrous Mg-Al-Ca-Na-K-rich silicate liquid and a Fe-Cr-Al-Ni-rich oxyarsenide liquid were unmixed. These two immiscible liquids were segregated and emplaced in veins and schlieren to consolidate as separate but intimately mixed rocks. The silicate liquid crystallized into plagioclase-and mica-bearing cordierite rocks, the oxyarsenide liquid unmixed again in chromite-rich and arsenide-rich fractions before consolidating as chromite-niccolite rocks.